The invention is generally directed to an apparatus useful in evaluating optical properties of a surface containing metallic particles such as a paint containing metallic flakes. In particular, this invention is directed to an apparatus useful for visually evaluating color properties of a metallic paint, consistent with instrumental measurements.
The paint industry often utilizes light-reflecting flaked pigments in paints (i.e., metallic paints) to obtain pleasing aesthetic effects. Paints containing light-reflecting flaked pigment of such materials as aluminum, bronze, coated mica and the like are characterized by a “two-tone” or “flip-flop” effect whereby the apparent color of the paint changes at different viewing angles. This effect is due to the orientation of the flakes in the paint film. Since the color of such metallic paints will apparently vary as a function of the angle of illumination and viewing, multiple angled visual and/or spectrophotometric readings are required to accurately characterize the paint. See, for example, U.S. Pat. No. 4,479,718 issued Oct. 30, 1984 to Alman, the disclosure of which is herein incorporated by reference.
In the manufacture of pigmented paint, it is commonplace that simultaneous visual and instrumental color measurements are used in shading or batch release decisions. Most often, this situation exists due to geographic separation of the decision-makers along with a lack of instrumental equipment for all parties. As a result, a disparity between visual and instrumental color assessment for metallic and pearl colors frequently occurs, because simultaneous assessments are made at different aspecular angles. These disparities lead to inconsistencies between a color process operator's (shader's) assessment of color, and the measurement of a process system, such as a multi-angle spectrophotometer. These disparities quite often increase paint manufacturing time and become costly barriers to product batch release decisions between manufacturers and their respective customers.
A typical solution to compensate for the above described problem is installation and utilization of visual viewing equipment with fixed positions of the observer, light sources and multiple fixed positions of the paint color sample panel. An example is the Gretag MacBeth Skylight (C S McCamy) which has three light sources, fixed viewing port and five sample positions to provide 15 aspecular viewing angles between 10 and 110 degrees. However, this type of observation equipment is generally large, expensive, not considered mobile, and therefore has not been widely used in the industry.
Therefore, there is still a need for a color evaluation apparatus, which is mobile, inexpensive, and provides multiple fixed positions of viewing relative to the paint color sample panel.